This application claims the priority of German patent document 103 00 539.0, filed 9 Jan. 2003, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a circuit and method for monitoring the resistance between the terminals of a direct voltage source, such as for example a battery and a first reference potential.
The book “Schutztechnik und Isolationsüberwachung [Protective technology and insulation monitoring]”, Wolfgang Hofheinz, 6th Edition, VDE, chapter 14.3.1 describes a circuit for monitoring the resistance and for detecting insulation faults between the terminals of a direct voltage source and a ground potential. This circuit uses in each case two measuring instruments which are each connected between a terminal of the direct voltage source and the ground potential and have an internal resistor Ri. It is thus possible to detect an insulation fault between the ground potential and the respective other terminal of the direct voltage source by reference to a voltage drop at the internal resistor Ri, which results from a flow of current from the one terminal to ground via the measuring instrument, and from ground to the other terminal via the insulation fault. A third measuring instrument is used to sense the potential difference between the two terminals of the direct voltage source.
The large number of measuring instruments required to implement the known circuit make it complicated and costly.
One object of the invention, therefore, is to provide a particularly simple and inexpensive circuit for monitoring the resistance between the terminals of a direct voltage source.
Another object of the invention is to provide a monitoring circuit which requires only a single measuring instrument.
These and other objects and advantages are achieved by the monitoring circuit and method according to the invention, in which the inputs of the single measuring instrument are each connected to the external connections of a voltage divider whose central connection is kept at the first reference potential. Simple circuit breakers are sufficient as switches for switching over between a measurement between the first terminal and the reference potential, the second terminal and the reference potential or between the two terminals. The resistors which are connected in series between the external connection and the central connection of the voltage divider are preferably identical.
In order to permit measurement even when there are high output voltages of the battery, a protective resistor is preferably connected in series in each connection between one of the external connections of the voltage divider and one of the inputs of the circuit, the protective resistors also in turn preferably being identical.
A control unit is preferably provided for automated resistance monitoring, for the purpose, inter alia, of opening and closing the first and second switches in phase opposition.
The resistors of the voltage divider and the protective resistors are preferably ohmic resistors; but under certain circumstances, resistors with a virtual component of the resistance value are also possible.
An inexpensive differential amplifier can be used as a measuring instrument. If it is connected to a second reference potential as a supply potential (which under certain circumstances can differ from the first reference potential) there is preferably provision of a test voltage source having two terminals: The first terminal is connected to a point on the circuit which connects the second reference potential, and the second terminal is connected via a third switch to one of the external connections of the voltage divider. By closing the third switch so that the potential of the second terminal of the test voltage source connected to the measuring instrument, it is possible to detect whether the voltage applied to the measuring instrument differs significantly from a known output voltage of the test voltage source. It is thus possible to infer a potential difference between the first and second reference potential.
Such a difference in potential may occur in particular if i) the switches, the resistors and the measuring instrument are combined in one structural unit, ii) the central connection of the voltage divider is connected by a first line to the first externally defined reference potential and iii) a voltage drop can occur on a second line which connects the abovementioned point with the first reference potential (outside the structural unit), and thus defines the second reference potential.
The circuit according to the invention is preferably used to sense insulation faults on a battery which is installed in a vehicle, in particular an electrically driven vehicle. In this case, the first reference potential is preferably the vehicle ground.
In order to sense the resistance between the battery terminals and the first reference potential, the first switch is preferably alternately closed and the second opened in order to sense a first voltage which is then applied to the measuring circuit. The first switch is then opened and the second switch closed in order to sense a second voltage which is applied, and an insulation fault is indicated if at least one of the sensed voltages exceeds a limiting value.
In order to preclude functional faults in the circuit for such a measurement, various test steps are preferably carried out.
A first such test step comprises opening all switches, sensing the voltage which is then applied to the measuring circuit and which should be zero at this time, apart from an offset of the measuring instrument, and detecting a fault in the circuit if the voltage exceeds a limiting value by virtue of the fact that it is, for example, negative or greater than 0.1 V.
A further test step comprises opening the first and second switches and closing the third switch in order to sense possible differences between the first and second reference potentials, and detecting a fault in the circuit if the voltage exceeds a limiting value, for example if it is outside 0±0.8 V.
By closing the first and second switches and opening the third, a fixed fraction of the terminal voltage of the battery is applied to the measuring instrument. This fraction should also lie within a setpoint range in order to preclude disruption to the battery or to the first or second switch.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.